Heat treatment of metals



Feb. 4, 1941. J A, COMSTOCK 2,230,484

HEAT TREATMENT 0F METALS' original Filed Aug; so, 1938 (Ittorneg Patented Feb. 4, 1941 HEAT TREATMENT OF METALS John A. Comstock, Toledo, Ohio, assignor to Surface Combustion Corporation, Toledo, Ohio, a corporation of New York Continuation of application Serial No. 227,487, August 30, 1938. This application March 5, 1940, Serial No. 322,321

9 Claims.

This application is a continuation of my application, Serial No. 227,487, led August 30, 1938. Reference is also made to my Patent No. 2,188,275 dated January 23, 1940, wherein the apparatus herein disclosed is claimed.

The present invention has for its object to provide for the heat treatment of steel articles and molybdenum high speed tool steel in particular in a manner to avoid decarburization, scaling and 10 hydrogen sorption.

The heat treatment of molybdenum high speed steel presents diiiiculties that are not encountered in the heat treatment of other high speed steels.

Thus, whereas tungsten high speed steel can be clean hardened with practically no decarburization in an atmosphere resulting from the partial combustion of hydrocarbon gases, molybdenum high speed steel must be coated With a refractory material such as borie oxide,water glass or the like, as otherwise it will become decarburized. It is accepted that tungsten high speed steel is not decarburized in such an atmosphere due to the formation of an impervious stable oxide film over the surface of the article, this lm being initially formed at low temperatures in the preheat furnace and remaining throughout the high heat furnace treatment. In the case of molybdenum high speed steel, however, such a protective oxide lm cannot be maintained at heat treating tem- 80 peratures since molybdenum oxide is extremely volatile at heat treating temperatures as is readily observed during forging, a bluish White fume resembling cigarette smoke coming oi.

The nearer such an atmosphere is regulated to 86 a non-oxidizing constituence, i. e., by increasing gas-air ratio, the more pronounced is the decarburization of the work. Consequently such atmospheres are limited to the production of a minimum degree of oxidation consistent with 40 maximum allowable decarburization on such steels as form self-protecting oxide films at the particular heat treating temperature.

In the course of my experiments leading up to the present invention, I heated specimens of 45 molybdenum high speed in an atmosphere of ordinary charcoal gas butA without satisfactory results. I also made numerous tests using tank gases including nitrogen, hydrogen, ammonia, propane, etc. Mixtures of 10% hydrogen and 50 90%' nitrogen, which were deoxidized by passing over iron lings at a temperature between 600 and 700 F. and dehydrated by activated alumina, decarburized molybdenum high speed steel at heat treating temperatures. Tank hydrogen 65 alone similarly deoxidized and dehydrated also caused decarburization and, furthermore, to a greater degree than in the case of the mixed gases.` Tank nitrogen dried and de oxidized as above did not decarburize the steel but the specimens were found to be etched with a crystalline pattern resulting apparently from a `slight oxidization on heating followed by vaporization of the resulting molybdenum oxide at high temperature. It became evident, therefore, that an inert gas, such as commercial nitrogen, could not be used for this purpose because it cannot be obtained suliiciently free from oxidizing gas even if itcould be maintained in such a condition during use and also that. hydrogen was an undesirable reducing gas for mixing with the inert nitrogen. These observations were conrmed by the results obtained when using ammonia.

As a result of my experiments, I have discovered and this forms the basis of the present invention that molybdenum high speed Isteel can be successfully heat treated in an atmosphere containing CO, CO2, N2 and H2 provided that the H2 content is kept down to a very low limit, namely, to an amount not in excess of three onehundredths (.03) partial pressure in atmospheres. In general it may be said that for best results, the protective atmosphere should be substantially free from water vapor;` that the relative proportions of carbon monoxideand carbon dioxide should be such as to be in equilibrium with the steel at the prevailing temperature; that the hydrogen content expressed in partial pressure in atmospheres should be less than three onehundredths (.03), and that the nitrogen content expressed in partial pressure in atmospheres should be less than eighty-live one-hundredths (.85).

It is known from the work of others, and notably Takahashi (sethe article, in English, entitled On'the equilibrium between austenite and carbon oxides by Genske Takahashi, published in the Science Reports of Tohokii Imperial University, Sendi, Japan, Vol. XV, what the ratio of the square of the. partial pressure in atmospheres of carbon monoxide divided by the partial pressure in atmospheres of carbon dioxide should be to be in equilibrium with steel ,of any particular carbon content at any particular temperature. Thus, lor a steel at a temperature of l500 F. and having a carbon content of eighttenths per cent the said ratio is in the neighborhood of 12 whereas for the same steel at a temperature of 220 F., the ratio is in the neighborhood of 100.

This is only another way of saying that at the 56 V in order to avoid decarburization in' the absence of scale. Tables containing this data are well known and widely used in the art. By referring to such tables it is possible to ascertain very quickly what the relative proportions of CO and CO2 should be to be in equilibrium at various temperatures and to determine whether or not these components will be decarburizing, neutral or carburizing when in contact with a high carbon steel at the temperatures selected.

At this point it may be stated that molybdenum high speed steel contains about 9% molybdenum, 4% chromium, 1% vanadium and .78% carbon and that such steel is today recommended to be hardened at from 2150 to 2250 F., the usual temperature being 2200o F. Consequently at a temperature in the neighborhood of 2200 F., the relative proportions of CO and CO2 expressed in partial pressures in atmospheres should be such v that the ratio of of hydrogen into the metal being treated. Moreover, I have found that such a furnace atmosphere does not place a time limit at which the work may be held at temperature as in the case when using other atmospheres since, as I have found, the work maybe left for periods of several hours without decarburizing or scaling. Obviously this degree of protection may well be the basis of a new order of physical properties of heat treated steels. l

The present invention also provides a simple and practical method and apparatus for making the said protective atmosphere. the protective atmosphere is produced by passing an oxygen-containing gas low in hydrogen or` hydrogen compounds (this gas being hereinafter sometimes termed fbase gas) through a bed or cblumn of externally heated carbonaceous material which has been so conditioned by heat treatment as to be substantially devoid of hydrogen o'r hydrogen-containing compounds whereby the amount of hydrogen in said reaction products is substantially dependent on the amount of hydrogen or hydrogen compounds initially contained in said base gas, the temperatureof said carbonaceous material and the rate of now of the base gas being socontrolled as to produce the required or desired relative proportions of CO and CO2 to insure that the ratio of v expressed in partial pressures in atmospheres shall be non-decarburizing with respect to the steel at the prevailing heat treating temperature. The raw carbonaceous material may be charcoal or coke or other equivalent material, charcoal being preferred. 'Ihe choice of base gas will be determined largely by its relatively low 'content the same comprising a furnace chamber I4 which Briefly stated,

mixtures of these gases may, of course, be used 6 to arrive at the desired nitrogen content in the final reaction products. l v

i In the accompanying drawing forming part of this specification, I have shown two forms of apparatus that may be used in the practice of the l0 present invention. Fig. 1 is a vertical section of a heat treating furnace embodying a built-in gas generator designed to operate on what I call the counterow principle. Fig. 2 is a Vertical sectional view of the same kind of a heat treating furnace but embodying a built-in gas generator designed to operate on what I call the parallel flow principle.

Like reference numerals indicate like parts in the two views.

In both views, the heat treating furnace is shown as of the muiile type. I0 indicates the muiile, the same being provided at its front end with the usual door Il throfu'gh which ythe work to be heat treated may be placed in and removed from the munie. The door is shown as provided with a glass-covered sight hole I2 to permit visual inspection of the work in the muilie. "I'he furnace setting for the muiile is generally indicated at I3,

mally be kept closed by .a cover I6. For reasons 4,0l

presently, appearing the hopper is provided with a vent generally indicated at. I1, the same being Vshown as comprising a pipe I8 provided Iwith a hand valve I9 for controlling the effective discharge arear of the vent.

45 In the form of apparatus shown in Fig. 1, the

hopper I5 delivers the carbonaeous material to a vertical retort 20 positioned within the furnace chamber I l to the end that it may be heated to the same, or substantially the same, temperature 5o as the munie I0. The retort is shown as long enough to extend downwardly out of the furnace chamber with its lower end normally closed by any suitable removable closure, such 'as gate valves 2 I, to permit removal of ash. Leading into 55 the lower end of the retort is a pipe 22 through which the base gas lhereinbefore mentioned is delivere'd to the retort for upward fiow through the heated carbonaceous material. Said pipe leads from a source of supply of such gas and is pro- 6b vided with a flow adjusting valve 23. f A gas connection 25 is provided between the muiile I0 and the retort 20. For reasons present- *ly appearing, said connection is a substantial dis-L tance upwardly from the discharge end of the 65 gas pipe 22 and is also a substantial distance downwardly from the top of the furnace chamber I4. With respect to the muille, said gas connection 25 is shown as at the upper rear corner thereof but inasmuch as a isdesirble that the 7e gas enter the muiile proper near the lfloor thereof,

there is provided in themuiile a depending baille 26 which serves to direct' the gas downwardly.

In the form of apparatus shown in Fig 2, the

retort to which the hopper l5 deliversis indicated v75 extends downwardly into the retort above and terminates a substantial distance above the gas .connection 25. Ash resulting from the combustion of the carbonaceous material may be removed from the lower end of the retort by way of the gas connection. However, it will be readily appreciated that the lower end of the retort might well be extended downwardly below the .gas connection to form a receptacle from which ash may be withdrawn.

In Fig. 1, the retort 20 may be considered as comprising two zones A and B. Zone A is the gas making portion of the retort and zone B is the place where the carbonaceous material is conditioned before moving into zone A. In Fig. 2, the zones which correspond to the zones A and B in Fig. l are indicated at A and B' respectively.

For temperatures above 1850 F. the retort in both forms of apparatus will be made of a heatconducting refractory which preferably will be silicon carbide. For temperatures below 1850", it may be made of heat-resisting alloy.

To produce the protective' gas with the gas generator shown in Fig. 1, the procedure is substantially as follows. the hopper vent I1 is open, the valve 23 in the base gas supply pipe 22 is opened to such degree that considerably more base gas will always be supplied to retort 20 than is required to meet the requirements of the muiiie I to .the end that some of said gas shall always ow through zone B (and finally out through the hopper vent I1) in a direction opposed to the downcoming carbonaceous material C whereby to prevent the Volatilesgiven off by the heated carbonaceous material in zone B from contaminating the reaction products produced in zone A. The same procedure is. followed with the gas generator shown in Fig. 2, it being understood that some of the bise gas from the discharge end of the supply pipe 22 will flow upwardly through zone B'. By thus continuously purging the zone (B or B) where the carbonaceous material is preheated, it will be readily appreciated that the hydrogen content ofthe reaction products entering the muiiie by way of 4the gas connection 25 (25') may be readily controlled within the limits hereinbefore mentioned by utilizing a base gas Whose ,contentof hydrogen or hydrogen-containing compounds is known.

The relativev proportions of CO and CO2 in the products of reaction are a function of the temperature to which the carbonaceousmaterial is heated and by maintaining the retort at the same or substantially the same' temperature las the muiile the ratio of expressed in partial pressures in atmospheres will be such as to be neutraler slightly carburizing to the steel. l Although for simplicity of construction it is preferred that the retort be located in the furnace chamber which serves `to heat the muie, nevertheless the retort and the gasconnection between the retort and muiile may be otherwise heated to the required temperature.

Having made sure ,that i In the heat treatment of tool steel it is customary to employ two furnaces, one of which is usually referred to as the preheat furnace and the other as the high heat or super heat furnace. In the case of molybdenum high speed steel the temperature of the preheat furnace will ordinarily be maintained at a temperature of about 1550 F. (the work being allowed to soak thoroughly at this temperature) and the super heat furnace at a temperature of about 2200* F. As an indication of the difference in gas composition of the reaction products entering the muiiie from the zone A (A) and those issuing from the hopper vent when the retort is maintained at the two temperatures mentioned and when the base gas is low pressure atmospheric air, the following examples are given:

EXAMPLE No. l.-Retort at 1550 F.

Hopper Muic gas gas Percent Percent 3.6 0.7 0.0 0.0 28.7 33.1 7.4 2.1 0.0 0.0 Balance Balance at 2200 F.

Hopper Muie gas gas Percent Percent 0.1 0.0 34. 1. 0. Balauc The so-called hopper gas, that is to say, the gas issuing from-the vent I1 is entirely unsuited for use a's a protective atmosphere for molybdenum high speed steel. The so-called muflie gas, however, that is to say, the gas which enters the muiiie I0 from the zone A(A) of the retort will protect not only vmolybdenum high speedsteel but also other high speed steels and, in fact, steels generallyA against decarburization, scaling and hydrogen sorption. In this connection it may be stated that furnace atmospheres produced as herein described are applicable to many heating operations carried out on metals, including and especially, annealing, normalizing, preheating, hardening, tempering, brazing, melting, forging, etc.

From the foregoing, it will be seen that although the invention is particularly concerned with a protective atmosphere for molybdenum high speed steel, nevertheless an atmosphere produced in accordance with the present invention is useful in the heat treatment of metals generally. Consequently, I wish it to be understood that no limitations are intended except as expressed in theappended claims.

What I claim is:

1. The method of heat treating articles of steel in a furnace chamber which comprises maintaining a retort for carbonaceous material at a temperature substantially the same as that at a point remote from said opening a sulcient volume of a reactive gas to insure simultaneous outflow of gas through the said opening and offtake whereby to insure that the gas which flows into said chamber by way of said oitake shall be substantially devoid of the said volatiles and vapors, the said reactive gas consisting essentially of components which will react with the heated carbonaceous material to produce oxides of carbon to the substantial exclusion of hydrogen components. 4

2. 'I'he method of heat treating articles of steel in a furnace chamber which comprises maintaining a body of carbonaceous material in an externally heated retort at a temperature not less than that of said chamber, said retort having an oftake delivering to said chamber and having an opening remote from said offtake for the inflow of carbonaceous material from an extraneous source of supply and for the outilow of volatiles and vapors ordinarily given olf by the incoming material on being heated, delivering to said retort at a point remote from said opening a sufficient volume of a reactive gas to insure simultaneous outflow of -gas through the said opening and otake whereby to insure that the gas which ows into said chamber by way of said offtake shall be substantially devoid of the said volatiles and vapors, the said reactive gas consisting essentially of components which will react with the heated carbonaceous material to produce oxides of carbon to the substantial exclusion of hydrogen components, and the gas which ilows to the furnace chamber from said retort being maintained during its flow at a temperature not less than that of said chamber.`

3. The method of supplying to a furnace chamber wherein steel articles are placed for heat treatment an atmosphere comprising oxides of carbon in substantial equilibrium with the steel at the temperature of saidchamber, said oxides being made by heating carbon in the presence of air in a retort in gas connection with said chamber and being substantially devoid of the volatile impurities given oil by ordinary commercial carbon when it is first heated, which method comprises maintaining said retort and chamber at substantially the same temperature, feeding said carbon into said retort from an external source of supply, continuously prevent-A ties against the incoming carbon, and flowing said air into the retort at a point where the carbon is substantially devoid of said impurities.

4. The method of heattreating steel articles in a furnace chamber that is in gas connection with a retort for carbonaceous material', which method comprises maintaining said retort andv chamber and gas connection at substantially the same` temperature, maintaining a supply of said material in said retort by feeding the material thereto from an extraneous source of supply, continuously preventing volatile impurities given of! by said material as it becomes heated from nowing to' said chamber by way of said gas connection by continuously drawing off gas from the retort against the incoming carbonaceous malterial, and delivering to said retort at a point where said material is substantially devoid of said impurities an oxygen containing gas to produce reaction products consisting essentially of carbon monoxide for flow rto said chamber by way of said gas connection.

.2,280,484 terial on being heated, delivering to saidv retort 5. The method of supplying to a furnace chamber wherein metal articles are placed for heat treatment an atmosphere consisting essentially of oxides of carbon substantially devoid of the volatile impurities given off by ordinary commercial charcoal when it is first heated, said oxides being made by heating said charcoal in an externally heated retort in the presence of air, which method comprises feeding the charcoal into the retort from an external source of supply, continuously preventing said impurities from ilowing to said gas connectionby continuously drawing ofi' gas from retort against the incoming charcoal, and flowing said air into the retort at a point where the charcoal is substantially devoid of said impurities.

6. The method of heat treating anticles of high speed steel in a furnace chamber which comprises maintaining a retort for carbonaceous material and said chamber at a temperature in the neighborhood of 2200 F., said retort having an offtake delivering to said chamber and having an opening remote from said oitake for rthe inflow of carbonaceous material from an extraneous source of supply and for the outflow of volatiles and vapors ordinarily given oil by the incoming material on being heated, delivering to said retort at a point remote from said opening a suilicient volume of a reactive gas to insure simultaneous outflow of gas through the said opening and offtake whereby to insure that the gas which flows into said chamber by way of said oiltake shall be substantially devoid of the said volatiles and vapors, the said reactive gas consisting essentially of components which will react with the heated carbonaceous material to produce oxides of carbon to the substantial exclusion of hydrogen components.

'7. The method of supplying to a furnace chamber wherein articles of high speed steel are placed for heast treatment an atmosphere comprising oxides of carbon in substantial equilibrium with the steel at the temperature of said chamber, said oxides being made by heating carbon in the presence of air in a retort in-gas connection with said chamber and being substantially devoid of the volatile impurities given off by ordinary commercial carbon when it is first heated, which methodcomprises maintaining said retort and chamber at a temperature in the neighborhood of 2200 F., feeding saidcarbon into said retort from an external source of supply, continuously preventing said impurities from flowing to said gas connection by continuously drawing off said impurities against the incoming carbon, and flowing said air into the retort at a point where the carbon is substantially devoid of said impurities.

8. The method of heat treating steel articles in a furnace chamberl that is in gas connection with a retort for carbonaceous material, which method comprises maintaining said retort and chamber and gas connection at a temperature not less .fthan 1550 F., maintaining a supply of said material in said retort by feeding the material -thereto from an extraneous source of supply, continuously preventing volatile impurities given on by said material as it becomes heated from flowing to said chamber by way of said gas connection by continuously drawing of! gas from the retort against the incoming carbonaceous material, and delivering to said retort at a point where said material is substantially devoid of said impurities an oxygen containing gas to produce reaction products consisting essentially of carbon monoxide for flow to said chamber by way of said gas connection.

9. The method of supplying to a furnace chamber wherein metal articles are placed for heat treatment an atmosphere consisting essentially of oxides of carbon substantially devoid of the volatile impurities given oi by ordinary commercial charcoal when ixt is ilrst heated, said oxides being made by heating said charcoal to a tem- 10 perature of from 1550 to 2200 F. in an exter- .where the'charcoal is substantially devoid of said impurities.

JOHN A. COMSTOCK. 

